[hotfix] Fix the parameter typo in java doc (#651)

Author: Xianxun Ye

flink-connector-mysql-cdc/src/main/java/com/ververica/cdc/connectors/mysql/source/MySqlSourceBuilder.java

@@ -35,7 +35,7 @@
  *
  * <pre>{@code
  * MySqlSource
- *     .<RowData>builder()
+ *     .<String>builder()
  *     .hostname("localhost")
  *     .port(3306)
  *     .databaseList("mydb")